1. Field of the Invention
The present invention relates to thin film coatings for optical components, and more particularly, to a method of reducing the absorptance of thin film optical coatings, especially applicable in the field of laser optics.
2. Description of the Prior Art
In optical systems such as high power lasers, for example, the high absorptance of thin film coatings applied to optical components has become a severe problem to overcome. High absorptance causes excessive heat which in turn causes distortion and frequent damage to such components, thus limiting the desired high power performance. It has been found that the absorption coefficient of a thin coating deposited on an optical component can be, and generally is, thousands of times greater than that of the same coating material in bulk form. Since such coatings are necessary to provide the desired optical properties, this problem has become severe. Even when the fraction of absorbed light is optically tolerable, it may cause an unsatisfactory degree of heating.
In conventional practice, methods of limiting absorptance have consisted of careful cleaning, removing impurities from materials used, paying strict attention to deposition conditions, and the like. However, the high remaining absorptance is due to other factors.
U.S. Pat. No. 4,115,163 to Gorina et al discloses a method of growing an epitaxial semiconductor film on a substrate without incorporating defects and impurities in the film, which comprises cleaning and heating the substrate before and during the epitaxial growth by means of irradiating the substrate with an intensive luminous flux from a powerful light source.
While not relating to optical coatings but treating an already-formed semiconductor film with means similar to that used in the present invention, U.S. Pat. No. 4,059,461 to Fan et al describes scanning the semiconductor film with a focused laser beam. The laser is made to heat the film above a certain temperature.
U.S. Pat. No. 4,131,487 to Pearce et al also describes a method of reducing defects in a semiconductor wafer which includes directing a high energy laser beam on the wafer, followed by heating the wafer to draw away defects. This is done before forming the solid-state electronic devices on the wafer.